Determination of the linear pressure coefficients of semiconductor bandgaps

Abstract

Measurement of the pressure dependence of the direct bandgap of a tetrahedral semiconductor shows a pronounced sublinearity. This is explained by the stiffening of the lattice under pressure, so that if the pressure is converted into change of lattice constants using a suitable equation of state, the relation between bandgap and lattice constant is found to be linear within experimental (and theoretical) error. However, fitting to the pressure data, many authors use a parabolic equation, Eg(P) = Eg(0) + aP - bP2. We show that this gives a systematic error in the determination of the linear term a which increases with the pressure range of the experiment. GaAs provides a typical example of the systematic error. We find that the true linear pressure coefficient of GaAs is near 11.5 meV/kbar in agreement with the recent low-pressure measurement of Perlin et al. rather than the long-accepted value of 10.7 meV/kbar.